(1) Field of the Invention
The present invention relates to a radiation imaging system which takes in incident radiation distribution and a nuclear medicine diagnosis instrument therefor and, in particular, relates to radio-graphing middle and high energy γ rays.
(2) Description of related art
As a radiation measuring apparatus applied in the medical field, a nuclear medicine diagnosis instrument such as a γ camera apparatus and a Single Photon Emission Computed Tomography (SPECT) apparatus is used. Those apparatuses are systems consisting of a detector which detects γ rays and a collimator which regulates the direction of the γ rays entering the detector. For the detector used in those apparatuses, a scintillator and a photomultiplier tube are nearly always combined together. A scintillator used in those apparatuses is generally a sheet of large crystal. An NaI (Tl) scintillator is widely used for a gamma camera apparatus and a SPECT apparatus. A scintillator emits light in response to incident radiations and amplifies those faint lights with a plurality of photomultiplier tubes to detect the radiations. Determination on detection positions of radiations is carried out by implementing arithmetic operations on the center of gravity from output signals of a plurality of photomultiplier tubes. That is, a scintillator is a measuring apparatus having positional analogue output. A scintillator has thickness around 9 mm to 15 mm. Due to scintillation light distribution as a result of reaction of the incident γ rays and statistical dispersion due to the arithmetic operation on the center of gravity with a photomultiplier tube, space resolution (intrinsic resolution) of the detector itself is limited to approximately 3 mm.
On the other hand, in the recent years, a γ camera (“Technology of Nuclear Medicine”, pp. 79-80, Japanese Society of Radiological Technology, Apr. 30, 2002, Ohm, Ltd.) with a CSI (Tl) scintillator of a discrete detection pixel unit corresponding with an image pixel or so-called pixel type and with photodiodes and a pixel type semiconductor detector (“Technology of Nuclear Medicine”, pp. 76-77, Japanese Society of Radiological Technology, Apr. 30, 2002, Ohm, Ltd.) capable of directly converting radiations into electrical signals and the like, that is, a nuclear medicine diagnosis instrument having positional digital outputs is being developed. A semiconductor detector with width of 1.4 mm configuring one detection pixel is also being developed. The width can be approximately 1 mm as the value corresponding to intrinsic resolution.
As a collimator, a parallel porous (parallel hole) collimator with a great number of elongated radiation passages arrayed in parallel to allow only incident γ rays perpendicular to the detector plane to pass and to obtain projection of a radio-graphed object.
In those gamma camera apparatuses and those SPECT apparatuses the shape of a collimator and thickness of a detector to be required are different depending on energy of γ-ray to be radio-graphed. For the parallel hole collimators for low energy γ-ray radio-graphing for 99mTc (140 keV) being γ-ray source used for general purposes, the hole diameter of the aperture passing γ rays is around 1 to 2 mm; a septum between radiation passages restraining incident components of γ rays into the adjacent radiation passage is thin with around 0.2 to 0.3 mm and length of the radiation passage is around 30 to 50 mm.
On the other hand, middle and high energy γ rays from a γ-ray source in the intensity of 67Ga (300 keV) and 131I (367 keV) provides significant γ-ray penetrating power. Therefore, thickness of a septum is required to be 1 to 2 mm for restraining penetration of incident γ rays to the adjacent radiation passage. In addition, in order to compensate sensitivity reduction due to decrease in the area of an opening of the collimator, the hole diameter of the aperture is set larger to the level of 4 mm. In addition, length of depth in the direction of the incident γ rays of the detector is desired to be made thick compared with that for low energy γ rays for attaining sufficient detection sensitivity. A generally-purposed NaI scintillator detector for low energy γ rays with 10-mm thickness and a one for high energy γ rays with 20-mm thickness are used.
Comparing a high resolution parallel hole collimator [LEHR collimator (LEHR: Low Energy High Resolution)] for low energy γ rays with a generally-purposed parallel hole collimator [HEGP collimator (HEGP: High Energy-General Purpose)] for middle and high energy γ rays, the hole shape of the aperture of the collimator is hexagon in the both cases. However, the HEGP collimator is extremely thicker than the LEHR collimator in septum, larger than the LEHR collimator in hole diameter and longer than the LEHR collimator in length of depth of detector. As for high energy γ-ray radio-graphing, length of depth of the detector gets longer. Therefore, intrinsic resolution of the detector itself gets worse. In addition, the hole diameter of the aperture of the collimator is wide. Therefore a point source is wider than the LEHR collimator in radio-graphing distribution to deteriorate space resolution significantly.
On the other hand, radio-graphing distribution of a volume source is expressed as overlapping of a set of point source response function of each aperture and is obtained as approximately uniform shape in a LEHR collimator. However, in the a HEGP collimator, the shadows of septa appear on the radio-graphing distribution and give rise to decrease in picture quality to constitute a critical obstacle for diagnostic imaging. In addition, the thick septum causes decrease in aperture of the collimator to drop sensitivity.
The problem of such parallel hole collimator is that the holes are essentially tightly arranged and septum thickness is required in order to collimate the incident γ rays. Since septum thickness is thin for γ-ray radio-graphing with the low energy, influence on space resolution falls within an ignorable range. But for γ-ray radio-graphing with high energy, the problems of that system become apparent, making application of the collimator difficult.
A pinhole collimator is a collimator not influenced by such septum thickness. An object of a pinhole collimator is to obtain high space resolution by enlarging radio-graphing. However, there are problems that a pinhole collimator is narrower than parallel hole collimator in radio-graphing view range; resolution is deteriorated in the periphery of view range to deform an image; and sensitivity is not good. The radio-graphing view range is determined by a range viewable from the detector plane through the holes in the aperture (viewing angle). The smaller viewing angle makes the view range narrower. In the case of radio-graphing a point source, the more obliquely enters the incident γ rays, the longer gets distance crossing the detector; and the wider becomes the radio-graphing distribution in the periphery of the radio-graphing view range. In order to reduce deformation in the periphery of radio-graphing view range due to oblique entrance into the detector, it is necessary to limit the aperture angle of the hole. Therefore, distance from the detector to the hole gets longer than in the case of the parallel collimator. The hole diameter of the hole is small and the distance is long. Therefore, in a lot of cases, the pinhole collimator is less intensified than the parallel collimator in sensitivity.
In the case of using the pinhole collimator in γ-ray radio-graphing with high energy, an increase in length of depth in the γ-ray incident direction of the detector intensifies deformation more in the periphery of the radio-graphing view range. Therefore, in order to improve that, the aperture angle in the collimator aperture is squeezed more to make distance between the aperture and the detector longer, resulting in the radio-graphing view range getting much smaller to decrease sensitivity. Moreover, in the case where the shape of a longitudinal section of the aperture is a knife edge shape, high-energy γ-ray penetration takes place in the portion where shielding gets weak in the periphery of the aperture; and the effective hole diameter gets larger than the physical hole diameter of the aperture. It is difficult to obtain desired high resolution even if enlargement in radio-graphing is carried out. That will squeeze the aperture angle as well to lengthen distance between the detector and the aperture to reduce sensitivity more.
In view of the above described problems, a subject of the present invention is to solve such problems and an object hereof is to provide a radiation imaging system, which is not influenced by a hole diameter of a collimator such as the above described conventional parallel hole collimator and septum thickness but has a view approximately equivalent to the size of the detector group and excellent resolution and sensitivity approximately equivalent to low energy γ-ray radio-graphing in middle and high energy γ-ray radio-graphing, and a nuclear medicine diagnosis instrument therefor.